The present invention relates to a seat belt retractor.
A seat belt retractor generally comprises a cylindrical bobbin or spool with a circular cross-section. Seat belt webbing is attached to and wound around the spool, and the spool is mounted on a spool shaft to be rotatable in the retractor to wind in webbing under the action of a retractor spring and to pay out webbing under the influence of relatively gentle forwardly directed movement of a vehicle occupant, for example to allow for normal movement associated with vehicle occupancy such as reaching forwards to activate a radio or a window, or to reach a glove compartment or door pocket. In a crash the more extreme momentum of the vehicle occupant activates a crash sensor which locks the spool against rotation and thus prevents forward motion of the vehicle occupant and injury due to the vehicle occupant colliding with the interior structure of the vehicle.
However, this sudden locking of the seat belt spool in a crash can itself sometimes cause injury to the vehicle occupant due to sudden impact of the torso with the belt webbing. This is particularly true in severe crashes. In recent years this problem has been recognized and some solutions proposed.
One known system is to effect force limiting by interposing a plastically or elastically deformable member in the force path. For example, a torsion bar may be incorporated into the spool. A torsion bar is made of steel that twists when high torque is applied and can rotate up to seven or eight times while remaining intact. It is connected to both ends of a split spool. One end of the spool is held against rotation by a locking ring engaged by a load bearing pawl when a crash sensor indicates danger. The other end of the spool is prevented from rotating by its connection to the locking ring via the torsion bar, but when the crash forces exceed a predetermined level, the torsion bar will twist and allow a limited controlled further pay-out of webbing, generally in proportion to the momentum of the vehicle occupant at the moment a crash condition is sensed. This pay-out lessens the otherwise severe restraining forces on the vehicle occupant at high speeds, especially during the initial moments of a crash.
However torsion bars are complex and expensive to form, and are not easily adaptable to different conditions, such as different vehicle characteristics, or different sizes and weights of vehicle occupants. There is a need for a less costly and less complex load limiting system.
According to the present invention there is provided A seat belt retractor comprising: a spool, mounted for rotation in the retractor for retraction or pay-out of seat belt webbing depending upon the rotation direction of the spool; a locking ring attached to one end of the spool; a means for locking the locking ring against rotation when a crash is sensed; and a means for force limiting allowing further pay-out of the seat belt webbing after the locking ring has locked, under the influence of a vehicle occupant""s forward momentum, wherein the means for force limiting comprises a length of wire, attached at one end to one of the spool or the locking ring, and wound at a distance spaced from said one end, on a pin fixed to the other of the spool or the locking ring, so that under the influence of crash forces above a predetermined value, the wire is pulled around the pin.